The present invention relates to a new and improved construction of optical smoke detector for the detection of a combustion process, specifically for a fire alarm.
The smoke detector of the present development is generally of the type comprising a radiation source which transmits radiation throughout a conical ring-shaped radiation region. A radiation receiver is arranged externally of the direct radiation region in the cone axis and receives radiation which is scattered by particles located in the radiation region. Such smoke detectors are typically used, for instance, for detecting and reporting a fire or the like.
An important problem which exists with such smoke detectors resides in maintaining as low as possible the irradiation of the radiation receiver when there is not present smoke in the radiation region, so that upon the presence of the smallest amount of scattered radiation, caused by smoke particles located in the scattered radiation region, there is produced a signal at the output of the radiation receiver. Such type smoke detector will respond to the smallest smoke concentration and will detect and signal the presence of smoke with increased sensitivity.
In practice, however, there is always present a certain level of spurious or disturbing radiation, preventing the attainment of this objective. Therefore, it is already known in the art to use baffles at the air inlet openings of the smoke detector for screening the spurious radiation which enters the smoke detector from the outside through the housing openings, and thus, reducing the spurious radiation level, but with the drawback that through this technique there is also slowed down the entry of air. A further known technique in this art is to modulate the radiation source and to tune the radiation receiver to the modulated radiation source such that the radiation receiver is preferably only sensitive to radiation whose modulation corresponds to that of the radiation source.
Yet, in the aforementioned manner there cannot be prevented that radiation emanating from the radiation source and scattered at the inner wall of the housing likewise impinges as spurious radiation at the radiation receiver. Such spurious radiation is processed by the receiver in the same manner as radiation which is really scattered at the smoke particles in the radiation region, since it possesses exactly the same modulation. In order to reduce this type of spurious radiation it has become known to the art to structure the inner surface of the housing of the smoke detector so as to be extensively radiation absorbent at least at the impingement locations of the direct radiation at the radiation receiver, for instance to structure such inner surface of the housing so as to be dull black, to provide it with ribs or to construct it as a radiation trap. A typical smoke detector of such type is disclosed in U.S. Pat. No. 3,185,975, which issued May 25, 1965 to Arlon D. Kompelien.
What is disadvantageous with this design of smoke detector is that dust tends to deposit with time upon the radiation absorbing elements, for instance upon the dull black surfaces or the edges of the mounted ribs. This deposited dust or similar contaminants increases the reflection capability and again annihilates the radiation absorbing effect. Such heretofore known smoke detectors therefore become increasingly susceptible with time to triggering false alarms as the deposition of dust at the inner surface of the fire detector housing increases.